Tin cans are normally produced by preliminary forming, deep drawing and smoothing. They have a desirable bright surface so that, after coating with a clear or opaque organic lacquer or printing of the outer surface, they are suitable as an attractive pack. The sequence of process steps involved in the production of tin cans normally comprises offwinding the strip of tin plate provided with a layer of protective oil from the coil, applying drawing lubricants, preliminary forming into a cup and deep drawing and smoothing to the final shape. In the deep drawing and smoothing steps, cooling lubricants, such as water or aqueous emulsions, are normally also used to facilitate the deep drawing process. After forming, residues of the protective oil and deep drawing lubricants and also metal dust are removed in a cleaning steep. After the cleaning step, the containers pass through one or more water rinsing stages and are then dried in a drying oven. This is followed by lacquering in one or more stages and by decorative printing of the outer surface. Accordingly, the metal surface has to be of such quality that the lacquer has sufficient adhesion and affords reliable protection against corrosion.
In the production of such cans, however, it has been found that rust spots can develop during or before drying, particularly if the cans are cleaned with an acidic medium (pH 3-5), too much water is retained in certain areas or if, during stoppage of the production line, the individual process steps are not completed sufficiently quickly. Rust spots can thus develop. They are visible through the lacquer and promote poor lacquer adhesion, so that the product packed in such a container soon becomes unfit for consumption.
According to the teaching of EP-B-161 667, this problem can be solved in non-tinned steel cans, so-called black plate cans, by treating the cans after the actual cleaning step with an aqueous corrosion-inhibiting solution which contains 10 to 5,000 ppm of aluminum ions, 10 to 200 ppm of fluoride ions and up to 1,000 ppm of ions of at least one of the metals titanium, zirconium and/or hafnium and which has a pH value of 2 to 5.5.
The problem addressed by the present invention was to provide a treatment solution, a concentrate for its preparation and a treatment process with which tin cans could be cleaned and provided with a surface protected against corrosion in a single treatment step so that corrosion of the surface before lacquering would be prevented and firm lacquer adhesion would be promoted.